David Parkhurst

Research Hydrologist

Short Biography

I have worked for the U.S. Geological Survey since high school. Before going to graduate school, I worked on calcite dissolution kinetics, sediment pore water chemistry, and geochemical modeling. I received a Master’s degree from Stanford University in 1985, and worked for the District Office in Oklahoma City on abandoned lead and zinc mines, the Roubidoux Aquifer, and a pilot NAWQA project. In 1989 I became chief of the Reactive-Transport Modelingproject in Denver.I develop geochemical reaction and transport models that can be used to investigate contaminant migration, acid mine drainage, nuclear waste disposal, and carbon sequestration.

Muller, A.B., Parkhurst, D.L., and Tasker, P.W., 1986, The use of the PHREEQE code in modelling environmental geochemical problems encountered in performance assessment modelling, in Symposium on Ground-water Flow and Transport Modelling for Performance Assessment of Deep Geologic Disposal of Radioactive Waste--A Critical Evaluation of the State of the Art: Sponsored by U.S. Department of Energy, Civilian Waste Management, May 20-21, 1985, Albuquerque, New Mexico.

My USGS Science Strategy Areas

Problem: To understand contaminant migration and natural geochemical processes, we need to be able to model the movement of substances undergoing biogeogeochemical reactions in groundwater systems. Models that simulate physical, chemical, and bio­logical processes during contaminant migration are an effective way to evaluate the designs of chemical and nuclear waste repositories; to mini­mize the expenses of field studies of contaminant movement and site remediation; to evaluate the aquifer vulnerability to contaminants; and to investigate rock-water interactions, such as the formation of ore deposits, in situ min­ing, CO2 sequestration, and the natural evolution of ground-water chemistry.

Objectives: The principal objectives of the project are (1) to develop gener­al-use computer models that can identify and simulate geochemical and biological processes in flowing ground-water systems, and (2) to apply these models in field investigations.

Approach: The approach of the project has been to develop a geochemical model that simulates a wide range of equilibrium and kinetic biogeochem­ical processes, followed by development of more complete models that in­clude the additional processes of flow and transport. These models that simulate reaction and transport are applied to field investigations to de­duce the important chemical and biological reactions and to determine rate expressions that describe these reactions.

Supported Software:

PHREEQC: Batch geochemical reaction model that has capabilities to (1) speciate a water analysis to calculate saturation indices; (2) calculate reactions in a beaker, including mineral dissolution/precipitation, gas phases, ion exchange, surface complexation, solid solutions, and general kinetic reactions, plus the effects of temperature and pressure; (3) one-dimensional transport, including advection, dispersion, diffusion, multicomponent diffusion, diffusion through surface layers, and simultaneous chemical reactions; and (4) inverse modeling, which calculates the geochemical reactions necessary to account for the change in composition from one water composition to another.

IPhreeqc: A module containing all of the capabilities of PHREEQC that is designed to be incorporated into other software, particularly reactive-transport models. IPhreeqc can be the geochemical calculation engine for applications that need results of geochemical reaction modeling. For example AMDTreat uses IPhreeqc to calculate the amount of base needed to treat acid mine drainage. The COM version of IPhreeqc can be used in Excel, Visual Basic, Python, Matlab, and other software that interfaces with Microsoft COMs.

PHAST: Reactive transport model based on PHREEQC and HST (Ken Kipp) that simulates groundwater flow, solute transport, and geochemical reactions. All of the reaction capabilities of PHREEQC are available.

Phast4Windows: Three-dimensional graphical user interface for PHAST. Model features are defined with spatial (not grid) coordinates. Features can be defined, viewed, and edited in the interface. ARCGIS coverages can be used to define zones in the model domain, for which model parameters can be defined. All data is readily accessible in a data tree and visualizations can be zoomed, panned, and 3D rotated. Serial or parallel versions of the code can be run from the interface.

Model Viewer: A version of the Model Viewer software (Hsieh and Winston) is maintained to visualize results of PHAST calculations. During a PHAST run, specified data are written to HDF (hierarchical data format) files, which can be visualized in three dimensions with Model Viewer.

NetpathXL: Inverse geochemical model developed by Plummer and others. NetpathXL uses Excel to define the concentrations from chemical analyses, while the inverse modeling is the same as the original Netpath program.